Surface modified nanoparticle and method of preparing same

ABSTRACT

The present disclosure relates to a nanoparticle containing at least one metal sulfide nanocrystal having a surface modified with a carboxylic acid, wherein the carboxylic acid has at least one aryl group. The present disclosure also describes a method of preparing the nanoparticle, the method consisting of: (a) providing a first solution having a first organic solvent, and a non-alkali metal salt and a carboxylic acid dissolved therein, wherein the carboxylic acid has at least one aryl group; (b) providing a sulfide material; and (c) combining the first solution and the sulfide material to form a reaction solution, thereby forming a nanoparticle containing at least one metal sulfide nanocrystal having a surface modified with the carboxylic acid, wherein the carboxylic acid has at least one aryl group.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/089,323,filed on Mar. 24, 2005, now pending, the disclosure of which isincorporated by reference in its entirety herein.

This application is related to commonly assigned, co-pending U.S. patentapplications:

Ser. No. 11/089,319 by Denisiuk et al., entitled “Polymer NanocompositeHaving Surface Modified Nanoparticles and Methods of Preparing Same”,and filed Mar. 24, 2005 (Docket 60555US002); and

Ser. No. 11/089,347 by Denisiuk et al., entitled “Method of PreparingPolymer Nanocomposite Having Surface Modified Nanoparticles”, and filedMar. 24, 2005 (Docket 60462US002).

FIELD OF THE INVENTION

The present disclosure relates to a surface modified nanoparticle, andparticularly, to a metal sulfide nanoparticle having a surface modifiedwith a carboxylic acid, wherein the carboxylic acid comprises at leastone aryl group. A method for preparing the nanoparticle is alsodisclosed.

BACKROUND

Nanocomposites are mixtures of at least two different components whereinat least one of the components has one or more dimensions in thenanometer region. Nanocomposites have found use in many applicationsbecause, for example, they exhibit properties attributable to each ofits components. One type of nanocomposite comprises nanoparticlesdistributed in an organic matrix such as a polymer. This type ofnanocomposite is useful in optical applications, wherein thenanoparticles are used to increase the refractive index of the polymer.The nanoparticles must be uniformly distributed with minimal coagulationwithin the polymer, such that the nanocomposite exhibits minimal hazedue to light scattering.

There is a need for suface modified nanoparticles that can be readilyprepared and that can be used to form nanocomposites suitable foroptical applications.

SUMMARY

The present disclosure relates to a nanoparticle comprising at least onemetal sulfide nanocrystal having a surface modified with a carboxylicacid, wherein the carboxylic acid comprises at least one aryl group. Thepresent disclosure also provides a method of preparing the nanoparticle,the method comprising (a) providing a first solution having a firstorganic solvent comprising a non-alkali metal salt and a carboxylic aciddissolved therein, wherein the carboxylic acid comprises at least onearyl group; (b) providing a sulfide material; and (c) combining thefirst solution and the sulfide material to form a reaction solution,thereby forming a nanoparticle comprising at least one metal sulfidenanocrystal having a surface modified with the carboxylic acid, whereinthe carboxylic acid comprises at least one aryl group. The nanoparticledisclosed herein may be readily prepared and may be useful innanocomposites for optical applications.

DETAILED DESCRIPTION

The present disclosure relates to a nanoparticle that comprises at leastone metal sulfide nanocrystal having a surface modified with acarboxylic acid, wherein the carboxylic acid comprises at least one arylgroup. The present disclosure also relates to a method of preparing thenanoparticle. In one embodiment, the nanoparticle may be prepared by themethod:

-   -   (a) providing a first solution of a first organic solvent        comprising a non-alkali metal salt and a carboxylic acid,        wherein the carboxylic acid comprises at least one aryl group        dissolved therein;    -   (b) providing a sulfide material; and    -   (c) combining the first solution and the sulfide material to        form a reaction solution, thereby forming a nanoparticle        comprising at least one metal sulfide nanocrystal having a        surface modified with the carboxylic acid, wherein the        carboxylic acid comprises at least one aryl group.        In another embodiment, the method may further consist of:    -   (d) precipitating the nanoparticle by adding a third solvent to        the reaction solution, wherein the third solvent is miscible        with the first organic solvent but is a poor solvent for the        nanoparticle;    -   (e) isolating the nanoparticle;    -   (f) optionally washing the nanoparticle with the third solvent;        and    -   (g) drying the nanoparticle to powder.

The first organic solvent may be any organic solvent capable ofdissolving the non-alkali metal salt and the carboxylic acid comprisingat least one aryl group, and it must also be compatible with the sulfidematerial to form the reaction solution in which the nanoparticles areformed. In one embodiment, the first organic solvent is a dipolar,aprotic organic solvent such as dimethylformamide, dimethylsulfoxide,pyridine, tetrahydrofuran, 1,4-dioxane, N-methyl pyrrolidone, propylenecarbonate, or mixtures thereof.

The non-alkali metal salt provides metal ions that combinestoichiometrically with the sulfide material to form the metal sulfidenanocrystals. The particular choice of non-alkali metal salt may dependupon the solvents and/or the carboxylic acid comprising at least onearyl group used in the method described above. For example, in oneembodiment, the non-alkali metal salt is a salt of a transition metal, asalt of a Group IIA metal, or mixtures thereof, because metal sulfidenanocrystals of these metals are easy to isolate when water is used asthe third solvent. Examples of transition metals and Group IIA metalsare Ba, Ti, Mn, Zn, Cd, Zr, Hg, and Pb.

Another factor that influences the choice of the non-alkali metal saltis the desired properties of the metal sulfide nanocrystals, andtherefore, the desired properties of the nanoparticles. For example, ifthe nanoparticles are to be used in nanocomposites for opticalapplications, then the non-alkali metal salt may be a zinc salt becausezinc sulfide nanocrystals are colorless and have a high refractiveindex. Examples of nanocomposites that the nanoparticles may be used inare described in Ser. No. 11/089,319 by Williams et al., entitled“Polymer Nanocomposite Having Surface Modified Nanoparticles and Methodsof Preparing Same”, and filed Mar. 24, 2005 (Docket 60555US002), thedisclosure of which is herein incorporated by reference; and in Ser. No.11/089,347 by Williams et al., entitled “Method of Preparing PolymerNanocomposite Having Surface Modified Nanoparticles”, and filed Mar. 24,2005 (Docket 60462US002), the disclosure of which is herein incorporatedby reference. For semiconductor applications, the non-alkali metal saltmay be a cadmium salt because cadmium sulfide nanocrystals can absorband emit light in useful energy ranges.

The carboxylic acid comprising at least one aryl group modifies thesurface of the at least one metal sulfide nanocrystal. The particularchoice of carboxylic acid comprising at least one aryl group may dependupon the solvents and the non-alkali metal salt used in the methodsdescribed above. The carboxylic acid comprising at least one aryl groupmust dissolve in the first organic solvent and must be capable ofsurface modifying the at least one metal sulfide nanocrystal that formsupon combination of the first solution with the sulfide material.Selection of the particular carboxylic acid comprising at least one arylgroup may also depend upon the intended use of the nanoparticles. Foruse in nanocomposites, the carboxylic acid comprising at least one arylgroup may aid compatibility of the nanoparticles with the organic matrixinto which they are blended. In one embodiment, the carboxylic acidcomprising at least one aryl group has a molecular weight of from 60 to1000 in order to be soluble in the first organic solvent and givenanoparticles that are compatible with a wide variety of organicmatrices.

In another embodiment, the carboxylic acid comprising at least one arylgroup is represented by the formula:Ar-L¹-CO₂H

-   -   wherein L¹ comprises an alkylene residue of from 1 to 10 C        atoms, and wherein the alkylene residue is saturated,        unsaturated, straight-chained, branched, or alicyclic; and    -   Ar comprises a phenyl, phenoxy, naphthyl, naphthoxy, fluorenyl,        phenylthio, or naphthylthio group.        The alkylene residue may be methylene, ethylene, propylene,        butylene, or pentylene. If the alkylene residue has greater than        5 C atoms, solubility in the first organic solvent may be        limited and/or surface modification may be less effective. The        alkylene residue and/or the aryl group may be substituted with        alkyl, aryl, alkoxy, halogen, or other groups. The carboxylic        acid comprising at least one aryl group may be 3-phenylpropionic        acid; 4-phenylbutyric acid; 5-phenylvaleric acid;        2-phenylbutyric acid; 3-phenylbutyric acid; 1-napthylacetic        acid; 3,3,3-triphenylpropionic acid; triphenylacetic acid;        2-methoxyphenylacetic acid; 3-methoxyphenylacetic acid;        4-methoxyphenylacetic acid; 4-phenylcinnamic acid; or mixtures        thereof.

In another embodiment, the carboxylic acid comprising at least one arylgroup is represented by the formula:Ar-L²-CO₂H

-   -   wherein L² comprises a phenylene or napthylene residue; and    -   Ar comprises a phenyl, phenoxy, naphthyl, naphthoxy, fluorenyl,        phenylthio, or naphthylthio group.        The phenylene or napthylene residue and/or the aryl group may be        substituted with alkyl, aryl, alkoxy, halogen, or other groups.        The carboxylic acid comprising at least one aryl group may be        2-phenoxybenzoic acid; 3-phenoxybenzoic acid; 4-phenoxybenzoic        acid; 2-phenylbenzoic acid; 3-phenylbenzoic acid;        4-phenylbenzoic acid; or mixtures thereof

In the first solution, useful weight ratios of the carboxylic acidcomprising at least one aryl group to the non-alkali metal salt are from1:2 to 1:200. In one embodiment, the mole ratio of the carboxylic acidcomprising at least one aryl group to the non-alkali metal salt is lessthan 1:10. The particular weight and mole ratios used will depend on avariety of factors such as the solubilities of the carboxylic acidcomprising at least one aryl group and the non-alkali metal salt, theidentity of the sulfide material, the reaction conditions, e.g.temperature, time, agitation, etc.

The sulfide material used in (b) provides sulfide thatstoichiometrically reacts with the non-alkali metal ions to form the atleast one metal sulfide nanocrystal. In one embodiment, the sulfidematerial comprises hydrogen sulfide gas that may be bubbled through thefirst solution. In another embodiment, the sulfide material comprises asecond solution of a second organic solvent containing hydrogen sulfidegas or sulfide ions dissolved therein, wherein the second organicsolvent is miscible with the first organic solvent. Useful secondorganic solvents are methanol, ethanol, isopropanol, propanol,isobutanol, or mixtures thereof. The second solution of sulfide ions maybe obtained by dissolution of a sulfide salt in the second organicsolvent; useful sulfide salts are an alkali metal sulfide, ammoniumsulfide, or a substituted ammonium sulfide. It is often useful to limitthe amount of sulfide material to 90% of the stoichiometric equivalentof the non-alkali metal ions. In one embodiment, the first solutioncomprises non-alkali metal ions dissolved therein, and the secondsolution comprises sulfide ions dissolved therein, and the mole ratio ofthe non-alkali metal ions to the sulfide ions is 10:9 or more.

The nanoparticle disclosed herein comprises at least one metal sulfidenanocrystal. In one embodiment, the metal sulfide nanocrystals aretransition metal sulfide nanocrystals, Group IIA metal sulfidenanocrystals, or mixtures thereof. In another embodiment, the metalsulfide nanocrystals comprise zinc metal sulfide nanocrystals. In yetanother embodiment, the mineral form of the zinc metal sulfidenanocrystals is sphalerite crystal form, because sphalerite crystal formhas the highest refractive index compared to other mineral forms of zincsulfide, and so is very useful in nanocomposites for opticalapplications.

The nanoparticle disclosed herein comprises at least one metal sulfidenanocrystal, and the exact number of nanocrystals may vary depending ona variety of factors. For example, the number of nanocrystals in eachnanoparticle may vary depending on the particular choice of thenon-alkali metal salt, the carboxylic acid comprising at least one arylgroup, or the sulfide material, as well as their concentrations andrelative amounts used in (a), (b), or (c). The number of nanocrystals ineach nanoparticle may also vary depending on reaction conditions used in(a), (b), or (c); examples of reaction conditions include temperature,time, and agitation, etc. All of these aforementioned factors may alsoinfluence shape, density, and size of the nanocrystals, as well as theiroverall crystalline quality and purity. The number of metal sulfidenanocrystals may vary for each individual nanoparticle in a givenreaction solution, even though the nanoparticles are formed from thesame non-alkali metal ions and sulfide material, and in the samereaction solution.

The at least one metal sulfide nanocrystal has a surface modified by thecarboxylic acid comprising at least one aryl group. The number ofsurfaces may vary depending on the factors described in the previousparagraph, as well as on the particular arrangement of nanocrystalswithin the nanoparticle if more than one nanocrystal is present. One ormore individual carboxylic acid molecules may be involved in the surfacemodification, and there is no limit to the particular arrangement and/orinteraction between the one or more carboxylic acid molecules and the atleast one metal sulfide nanocrystal as long as the desired properties ofthe nanoparticle are obtained. For example, many carboxylic acidmolecules may form a shell-like coating that encapsulates the at leastone metal sulfide nanocrystal, or only one or two carboxylic acidmolecules may interact with the at least one metal sulfide nanocrystal.

The nanoparticle disclosed herein may have any average particle sizedepending on the particular application. As used herein, averageparticle size refers to the size of the nanoparticles that can bemeasured by conventional methods, which may or may not include thecarboxylic acid comprising at least one aryl group. The average particlesize may directly correlate with the number, shape, size, etc. of the atleast one nanocrystal present in the nanoparticle, and the factorsdescribed above may be varied accordingly. In general, the averageparticle size may be 1 micron or less. In some applications, the averageparticle size may be 500 nm or less, and in others, 200 nm or less. Ifused in nanocomposites for optical applications, the average particlesize is 50 nm or less in order to minimize light scatter. In someoptical applications, the average particle size may be 20 nm or less.

Average particle size may be determined from the shift of the excitonabsorption edge in the absorption spectrum of the nanoparticle insolution. Results are consistent with an earlier report on ZnS averageparticle size—(R. Rossetti, Y. Yang, F. L. Bian and J. C. Brus, J. Chem.Phys. 1985, 82, 552). Average particle size may also be determined usingtransmission electron microscopy.

The nanoparticles may be isolated by using any conventional techniquesknown in the art of synthetic chemistry. In one embodiment, thenanoparticles are isolated as described in (d) to (g) above. The thirdsolvent is added to the reaction solution in order to precipitate thenanoparticles. Any third solvent may be used as long as it is a poorsolvent for the nanoparticles and a solvent for all the other componentsremaining in the reaction solution. A poor solvent may be one that candissolve less than 1 weight % of its weight of nanoparticles. In oneembodiment, the third solvent is water, a water miscible organicsolvent, or mixtures thereof. Examples of water miscible organicsolvents include methanol, ethanol, and isopropanol.

The nanoparticles may be isolated by centrifugation, filtration, etc.,and subsequently washed with the third solvent to remove non-volatileby-products and impurities. The nanoparticles may then be dried, forexample, under ambient conditions or under vacuum. For someapplications, removal of all solvents is critical. For nanocompositesused in optical applications, residual solvent may lower the refractiveindex of the nanoparticles, or cause bubbles and/or haze to form withinthe nanocomposite.

The examples described below are presented for illustration purposesonly and are not intended to limit the scope of the invention in anyway.

EXAMPLES

Nanoparticles and Their Preparation

Preparation of H2S in Isopropanol

A solution containing 0.200 g of zinc acetate dihydrate (0.00091 mole)in 10 mL dimethylformamide (DMF) was prepared. Another solutioncontaining H₂S in isopropanol (IPA) was prepared by passing a stream offine bubbles of the H₂S gas through the IPA for 24 hours, after whichtime it was assumed that the solution was saturated. The zinc acetatesolution was titrated with the H₂S solution until lead acetate paperindicated the presence of excess H₂S. From this titration was determinedthe volume of the H₂S solution having 0.00083 mole of H₂S (10 mole %excess of zinc over H₂S). In order to prepare solutions for thefollowing examples, this determined volume was multiplied by 10 and thenIPA was added to make a total volume of 50 mL.

Nanoparticle NP-1

A solution was prepared by dissolving 2.0 g of zinc acetate dihydrate(0.0091 mole) and 0.06 g of 2-phenoxybenzoic acid in 40 mL of DMF. Thiswas poured into 50 mL of the H₂S solution described above, containing0.0083 mole of H₂S in IPA, wth strong stirring agitation. To theresulting mixture was added with stirring 100 mL of water. The resultingmixture was allowed to stand at ambient conditions. A precipitate wasformed over a day and was separated by centrifugation and washed withwater and IPA. After drying overnight in a vacuum desiccator, a smallamount of the solid was dissolved in DMF using ultrasonic agitation.This solution was examined using UV-VIS spectroscopy, and a shoulder onthe absorption curve occurred at 290 nm, corresponding to an averageparticle size of 3.0 nm. Preparation of NP- I was repeated and theaverage particle size was 3.6 nm.

Nanoparticles NP-2 to NP-17

Nanoparticles NP-2 to NP- 17 were prepared as described for NanoparticleNP- 1, except that different carboxylic acids were used. The amount ofthe carboxylic acid was 0.06 g in each example, therefore the mole ratioof carboxylic acid to zinc acetate varied. A summary of thenanoparticles is listed in Table 1. The mole ratios of carboxylic acidto zinc acetate ranged from 0.022 to 0.048, and the average particlesizes ranged from 3 to 8 nm. TABLE 1 Mole Ratio of Average Nano- MW ofCarboxylic Particle parti- Carboxylic Acid to Zinc Size cle CarboxylicAcid Acid Acetate* (nm) NP-1 2-phenoxybenzoic acid 214 0.03 3.0, 3.6NP-2 3-phenylpropionic acid 150 0.044 4.5 NP-3 2-phenylbutyric acid 1640.04 3.8 NP-4 4-phenylbutyric acid 164 0.04 4.0 NP-5 2-naphthoxyaceticacid 202 0.032 3.2 NP-6 3-phenoxypropionic acid 166 0.04 5.0 NP-71-naphthylacetic acid 186 0.035 4.6 NP-8 triphenylacetic acid 288 0.0234.0 NP-9 5-phenylvaleric acid 178 0.037 4.2 NP-10 benzoic acid 136 0.048NM NP-11 phenoxyacetic acid 152 0.043 NM NP-12 2-phenoxypropionic acid166 0.04 NM NP-13 3-phenylbutyric acid 164 0.04 NM NP-142-phenoxybutyric acid 180 0.037 NM NP-15 2-methoxyphenylacetic 166 0.04NM acid NP-16 3,3,3-triphenylpropionic 302 0.022 NM acid NP-174-phenylcinnamic acid 240 0.027 NMNM = not measured*MW of zinc acetate is 219Dependence of Average Particle Size on the Concentration of CarboxylicAcid

The dependence of average particle size on the concentration ofcarboxylic acid was determined for NP-9, NP-1 and NP-4. The weightpercent concentration of each carboxylic acid relative to zinc acetatewas varied between 0.50 and 50.00 weight %, and the results are shown inTable 2. The results show that only small changes in average particlesizes for NP-9, NP-1 and NP-4 as a function of % of carboxylic acid wereobserved. TABLE 2 Weight % of Carboxylic Average Particle Acid Relativeto Zinc Size (nm) Acetate NP-9 NP-1 NP-4 50.00 3 2 2.2 33.33 3 2.2 39.09 3.8 3.8 3.8 0.50 3.8 4.5 3.8Dependence of Average Particle Size on Temperature

The dependence of average particle size on temperature was determinedfor NP-2. The temperature of the mixture containing zinc acetate,3-phenylpropionic acid, and H₂S was varied between −20 and 20° C., andthe results are shown in Table 3. The results show that only a smallaverage particle size change as a function of temperature was observedfor NP-2, and all were within the desired range. TABLE 3 AverageParticle Size Temperature (° C.) (nm) −20 1.6 5 3.0 20 3.7

1. A method of preparing a nanoparticle, the method comprising: (a)providing a first solution having a first organic solvent comprising anon-alkali metal salt and a carboxylic acid dissolved therein, whereinthe carboxylic acid comprises at least one aryl group; (b) providing asulfide material; and (c) combining the first solution and the sulfidematerial to form a reaction solution, thereby forming a nanoparticlecomprising at least one metal sulfide nanocrystal having a surfacemodified with the carboxylic acid, wherein the carboxylic acid comprisesat least one aryl group.
 2. The method of claim 1, wherein the firstorganic solvent is a dipolar, aprotic organic solvent.
 3. The method ofclaim 1, wherein the first organic solvent is dimethylformamide,dimethylsulfoxide, pyridine, tetrahydrofuran, 1,4-dioxane, N-methylpyrrolidone, propylene carbonate, or mixtures thereof.
 4. The method ofclaim 1, wherein the non-alkali metal salt is a salt of a transitionmetal, a salt of a Group IIA metal, or mixtures thereof.
 5. The methodof claim 1, wherein the nanoparticle has an average particle size of 50nm or less.
 6. The method of claim 1, wherein the carboxylic acidcomprising at least one aryl group has a molecular weight of from 60 to1000.
 7. The method of claim 1, wherein the weight ratio of thecarboxylic acid comprising at least one aryl group to the non-alkalimetal salt is from 1:2 to 1:200.
 8. The method of claim 1, wherein thesulfide material comprises hydrogen sulfide gas.
 9. The method of claim1, wherein the sulfide material comprises a second solution having asecond organic solvent comprising hydrogen sulfide gas or sulfide ionsdissolved therein, wherein the second organic solvent is miscible withthe first organic solvent.
 10. The method of claim 9, wherein the firstsolution comprises non-alkali metal ions dissolved therein, and thesecond solution comprises sulfide ions dissolved therein, and the moleratio of the non-alkali metal ions to the sulfide ions is 10:9 or more.11. The method of claim 10, wherein the second solution comprises asulfide salt dissolved therein, wherein the sulfide salt comprises analkali metal sulfide, ammonium sulfide, or a substituted ammoniumsulfide.
 12. The method of claim 9, wherein the second organic solventis methanol, ethanol, isopropanol, propanol, isobutanol, or mixturesthereof.
 13. The method of claim 1 further comprising: (d) precipitatingthe nanoparticle by adding a third solvent to the reaction solution,wherein the third solvent is miscible with the first organic solvent butis a poor solvent for the nanoparticle; (e) isolating the nanoparticle;(f) optionally washing the nanoparticle with the third solvent; and (g)drying the nanoparticle to powder.
 14. The method of claim 13, whereinthe third solvent comprises water, a water miscible organic solvent, ormixtures thereof.